This invention relates generally to photosensor arrays used for optical image scanners and cameras and more specifically to line arrays commonly used for optical image scanners.
Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be a separate device or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose device. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices.
In general, image scanners use an optical lens system to focus an image onto an array of photosensors. Photosensor arrays typically have thousands of individual photosensitive elements. Each photosensitive element, in conjunction with the scanner optics system, measures light intensity from an effective area on the document defining a picture element (pixel) on the image being scanned. Optical sampling rate is often expressed as pixels per inch (or mm) as measured on the document (or object, or transparency) being scanned. Optical sampling rate as measured on the document being scanned is also called the input sampling rate.
Photosensor arrays for image scanners commonly have three or four rows of sensors, with each row receiving a different band of wavelengths of light, for example, red, green and blue. Each row may be filtered, or white light may be separated into different bands of wavelengths by a beam splitter. Each row of sensors receives light from one line on the document, called a scanline. Typically, the pitch (spacing of individual photosensor elements) is the same for each row, and typically the pitch is set to provide a specified native input sampling rate.
The native input sampling rate is determined by the optics and the pitch of the individual sensors. A scanner operator may select a sampling rate that is less than the native input sampling rate by simply dropping selected pixels, or by using digital resampling techniques. Alternatively, a scanner operator may select a sampling rate that is greater than the native input sampling rate where intermediate values are computed by interpolation. Typically, all the charges or voltages are read from the photosensor array, and are then digitized, and then subsampling or interpolation is performed on the resulting digital pixel data.
Common photosensor technologies include Charge Coupled Devices (CCD), Charge Injection Devices (CID), Complementary-Metal-Oxide (CMOS) devices, and solar cells. Typically, for a CID or a CMOS array, each photosensitive element is addressable. In contrast, CCD line arrays commonly serially transfer all the charges, bucket-brigade style, from each line of photosensitive elements to a small number of sense nodes for conversion of charge into a measurable voltage. The present patent document is primarily concerned with photosensor arrays having serial charge shift registers, also called serial readout registers.
For typical CCD line arrays, the sensors are exposed to light for an exposure time; the charges are transferred in parallel, through transfer gates, to charge shift registers; the charges are serially shifted to sense nodes; and the charges are measured (analog-to-digital conversion). Let N=number of sensors per row, Ts=shift time to shift a charge from one stage in a charge shift register to the next stage, and Tc=analog-to-digital conversion time (including amplifier delay) for one voltage measurement. After exposure, the total time required to process one scanline is then approximately N*(Ts+Tc). If N increases, and all other parameters remain constant, then the time required to process each scanline increases. Typically, one scanline is being exposed while the previous scan line is being converted, and typically exposure time is greater than or equal to processing time for one line.
For the retail image scanner market, native input sampling rates have doubled about every three years. Image scanners are available that have a native input sampling rate that far exceeds what is necessary for many tasks. For example, the optical sampling rate needed for optical character recognition (OCR) is often much less than the native input sampling rates in commonly available image scanners. For a task such as OCR, the data from the scanner is commonly subsampled, so that, for example, three-fourths of the data is simply discarded. However, time is still required to shift and convert all the charges, most of which are not used.
There is need for decreasing the scan time for tasks that require an optical sampling rate that is less than the native input sampling rates.
An embodiment of a method for scanning comprises: exposing a first linear array of photodetector elements to light for a first exposure time to acquire first, second, third and fourth accumulated charges corresponding to charges of a first, second, third and fourth group of the photodetector elements, respectively; transferring the first, the second, the third and the fourth accumulated charges to first, second, third, and fourth stages, respectively, of a charge shift register; exposing the first linear array to light for a second exposure time to acquire fifth, sixth, seventh and eighth accumulated charges corresponding to charges of the first, second, third and fourth groups, respectively; and transferring the fifth, sixth, seventh and eighth accumulated charges to fifth, sixth, seventh, and eighth stages, respectively, of the charge shift register to interleave the first and second accumulated charges with the seventh and eighth accumulated charges.
Approximately one-fourth of the first scanline is processed (shifting, amplifying, and analog-to-digital conversion) during exposure of the second scanline. Then, for the second scanline, charges from four adjacent sensor elements are accumulated into one charge shift register stage, resulting in the second scanline interleaved with a portion of the first scanline. The process is repeated, resulting in up to four scanlines partially interleaved in the charge shift register. The portion of the charge shift register closest to the analog-to-converter is essentially fully interleaved. With interleaving and combining of charges, essentially all of the data from the analog-to-digital converter is used. As a result of reduced exposure time, and more efficient use of conversion time, the overall time required to acquire just the data that is needed is substantially reduced. For example, the overall time required to acquire data at one-fourth the native input sampling rate is reduced to almost one-fourth the time required to acquire data at the native input sampling rate.